Photoemission of electrons from a condensed phase can occur even when the absorbed photon energy is lower than the ionization potential of that phase. In such cases, the mechanisms of photoionization may involve interactions between excited states, excited state-photon absorption phenomena, or multiple photon absorption, giving rise to superlinear intensity dependence of photoelectron yields.
One known apparatus for measuring ps optical pulses is known as a Streak camera. However, it is very costly, and its resolution is about 3-5 ps, so that events lasting 1 ps or less, down into the femtosecond (fs) range, can not be measured.
A less expensive commercially available technique embodied in the (INRAD Autocorrelator available from Interactive Radiation, Inc., of Northvale, N.J.) is based on the superlinear characteristics of certain crystals. In these crystals, light of long wavelength is converted into light of shorter wavelength with an efficiency that varies with light intensity. The measuring device is referred to as an autocorrelator. Using this device, the light pulse width may be determined. There are some drawbacks with this device: (1) The crystals will only work in a limited range of optical wavelengths. They are not suitable for measuring light in the blue spectrum region, for example, or light that will produce fluorescence in the crystal. (2) The angle of polarization of the incident light must be precisely oriented with respect to the optical axes of the crystal, if maximum efficiency is to be realized. The efficiency of conversion accuracy of modeling one pulse shape falls off rapidly with a misorientation. (3) The incoming light of long wavelength must be diverted from the path of the outgoing light of shorter wavelength in order to avoid overloading the response of a photomultiplier that measures the efficiency of conversion to the shorter wavelength. (4) The auto-correlator is still a relatively expensive device.
Several recent papers have been published describing high speed circuit measurements using photoemission sampling. See Springer Series in Chemical Physics 46; Ultrafast Phenomena V Proceedings 5th Optical Soc. America Topical Meeting; Edited by G.R. Fleming & A.E. Siegman; Springer-Verlag Berlin Pub. Co., 1986, pages 123-130. These papers describe associating a sample metal electrode with a circuit part to the measured, and impinging on the metal electrode 500 fs optical pulses. This causes the generation of photoelectrons which are accelerated by an electric field between the sample and an electron collector. Useful information is obtained by measuring at the collector the change of electric current. However, the time resolution of this technique is limited by the transit time of the electrons from the sample surface to the collector. Since it is difficult to make collector-emitter spacings under one micron, measurement of sub-ps wide pulses cannot be done by this known technique.